Intertherm furnace problems

You may have to be VERY careful how you fix that piping and not alter it. Miller/Intertherm especially their mobile home or pre fab home units are very specific to using their concentric kit. They finely tune the combustion air supply and pressure switch settings due to them being so compact. I would want to get the install manual and see what methods they have for venting it. You may be able to go with a conventional 2 pipe thru the roof arrangement and get rid of that mess but you need to follow the original install info if they allow a 2 pipe setup.